Buffer layers play a key role in high-temperature superconductor (HTS) materials, particularly in recently developed second-generation rare-earth-barium-copper-oxide (REBCO), especially yttrium-barium-copper-oxide (YBCO) wire technology. The purpose of the buffer layers is to provide a continuous, smooth and chemically inert surface for the growth of the YBCO film, while transferring the biaxial texture from the substrate to the YBCO. Buffer layers are important for preventing metal diffusion from the substrate into the superconductor, and as oxygen diffusion barriers. Multi-layer architectures have been developed that also provide mechanical stability and good adhesion to the substrate.
The following patents provide ample background teaching and are incorporated herein by reference:
U.S. Pat. No. 6,663,976 issued on Dec. 16, 2003 to D. B. Beach, J. S. Morrell, M. Paranthaman, T. Chirayil, E. D. Specht, and A. Goyal, “Laminate articles on biaxially textured metal substrates”.
U.S. Pat. No. 6,399,154 issued on Jun. 4, 2002 to R. K. Williams, M. Paranthaman, T. G. Chirayil, D. F. Lee, A. Goyal, and R. Feenstra, entitled “Laminate Article”.
U.S. Pat. No. 6,440,211 issued on Aug. 27, 2002 to D. B. Beach, J. S. Morrell, M. Paranthaman, T. G. Chirayil, E. D. Specht, and A. Goyal, entitled “Method of Depositing Buffer Layers on Biaxially Textured Metal Substrates”.
U.S. Pat. No. 6,235,402 issued on May 22, 2001 to S. S. Shoup, M. Paranthaman, D. B. Beach, D. M. Kroeger, and A. Goyal, entitled “Buffer Layers on Biaxially Textured Metal Substrates”.
U.S. Pat. No. 6,270,908 issued on Aug. 7, 2001 to R. K. Williams, M. Paranthaman, T. G. Chirayil, D. F. Lee, A. Goyal, and R. Feenstra, entitled “Rare Earth Zirconium Oxide Buffer Layers on Metal Substrate”.
U.S. Pat. No. 6,077,344 issued on Jun. 20, 2000 to S. S. Shoup, M. Paranthaman, D. B. Beach, D. M. Kroeger, and A. Goyal, entitled “Sol-gel Deposition of Buffer Layers on Biaxially Textured Metal Substances”.
U.S. Pat. No. 6,150,034 issued on Nov. 21, 2000 to M. Paranthaman, D. F. Lee, D. M. Kroeger, and A. Goyal, entitled “Buffer Layers on Rolled Nickel or Copper as Superconductor Substrates”
U.S. Pat. No. 6,156,376 issued on Dec. 5, 2000 to M. Paranthaman, D. F. Lee, D. M. Kroeger, and A. Goyal, entitled “Buffer Layers on Metal Surfaces Having Biaxial Texture as Superconductor Substrates”.
U.S. Pat. No. 6,159,610 issued on Dec. 12, 2000 to M. Paranthaman, D. F. Lee, D. M. Kroeger, and A. Goyal, entitled “Buffer Layers on Metal Surfaces Having Biaxial Texture as Superconductor Substrates”.
It is important that the highly aligned buffer materials are matched in both the lattice constant and thermal expansion to the biaxially textured substrate and the YBCO layer. The buffer layers should be smooth, continuous, crack-free and dense. Even though the buffer layers are much thinner than the YBCO layer, buffer deposition cost is a substantial part of the total conductor cost. Hence, there is a need for the development of economically feasible, efficient, scaleable, high rate, high quality buffer layers and associated deposition processes.
When growing an epitaxial oxide film on a metal surface it is essential to avoid oxide formation before the nucleation of the layer is complete. For example YBCO is typically grown in an atmosphere containing 100 ppm or more of oxygen at 700-800° C. Under such conditions Ni and W will form various native oxides on a NiW surface, i.e. at PO2 levels below the required PO2 levels to oxidize Ni or W. Most of such oxide layers do not offer the bi-axial cubic texture required for high critical currents in the HTS layer. However, the ability of certain oxide films to be grown in very low oxygen partial pressures, i.e. below the oxidation of Ni and even W can be utilized. A thin seed layer is deposited first, to subsequently allow the growth of the full buffer and finally the YBCO processing at higher oxygen levels.
Oxide diffusion in most films is rather fast compared to cation diffusion. In reality the oxidation of the metal substrate cannot be suppressed completely. Thin homogeneous oxide layers are observed after final processing of the conductor without negative impact on the performance. However the buffer stack controls the amount and the time when the oxide layer is formed at the interface between the buffer and the substrate. Uncontrolled growth of substrate oxides can result in multiple failures. Rapid and inhomogeneous oxide growth can penetrate the buffer layers and cause a break down of its barrier properties. For example, when excessive NiO is formed the full buffer stack delaminates from the metal template due to volume expansion of, for example, Ni to NiO.
Fundamental understanding of the oxygen diffusion through oxide buffers is under study. Even though it is possible to grow oxide buffers directly on textured metal surfaces under suitable reducing conditions, oxygen penetrates through the buffer layers such as yttrium oxide, cerium oxide, and yttria-stabilized zirconia (YSZ) to the metal oxide interface during the YBCO processing. This is mainly because the oxygen diffusivity of these buffers at 800° C. is in the range of 10−9 to 10−10 cm2/sec. The diffusion is more rapid in structures that are more prone to the occurrence of defects. Among fastest oxygen diffusion is that of YSZ, which has a fluorite structure with a high probability of oxygen vacancies, providing easier routes for oxygen transport. Grain boundaries, pinholes and particulates can also provide short circuit diffusion paths in these films.
Referring to FIG. 1, some commonly used architectures begin with a starting template such as a rolling-assisted biaxially textured substrate (RABiTS) 1 comprising Ni—W (3 or 5 at. %). Layers include, for example, a buffer layer 2 of Y2O3 (75 nm), a barrier layer 3 of YSZ (75 nm), a capping layer 4 of CeO2 (75 nm), and a layer of REBCO 5. RE is Y or another rare earth element alone or doped with another rare earth element. Layer 4 could alternatively comprise a ferroelectric, piezoelectric or another semiconductor material. The buffers are generally deposited using physical vapor deposition (PVD) techniques.
The thickness of the YSZ layer is determined by the oxidation of the underlying substrate/buffer interface during the buffer deposition and the subsequent processing of the YBCO layer in highly oxidizing atmosphere. The cost of the buffer layer deposition corresponds to the total processing time. Thus it is necessary to develop a very fast deposition process (>10 Å/s) if thick layers (˜200 nm) are needed. Fast deposition rates impact the morphology and density of the growing film, which potentially increases the required thickness again. There is a need to reduce the required thickness and/or reduce the number of buffer layers in order to relax the requirements on the deposition rates and allow for a significant cost reduction.